Wbulk Wayne Like Olddog, been there done that comes to mind. If you are looking for a little better performance out of your cobra with your setup and cruising, you should look at what we are telling you. You don't have the TORQUE to have this car cruise at 1,800 rpms, it will lug and end up breaking your trans or clutch disc. If you check around with the Mustang guys, you will see that alot of them went from the 3.55 to 3.90 or 4.10 with the over drive trans The 3.55 was installed in the cars for the government, with CAFE, Emmissions, and MPG having to meet a certain number. That 270HP motor, the HP is at the crank shaft, not the rear tires, You are looking at about 185hp to the rear tires. I understand what you are trying to do, but in real life the computors just spit out info that may or may not work. It's the differents between theory and real world. The .68 is a large drop from 4th gear. If you do 70/30 of local (70%) to highway(30%) 3.73 or higher would be alot better choice. I like the 3.90's myself. We ran them in a fox body car and they gave the best of both worlds. Strong accelleration and good cruising mileage. Car ran 13.9 at the track. I am done.
You might want to check over at one of the mustang clubs for more info on this. Rick L.